1. Field of the Invention
The present invention relates to fluid droplet generation, and more particularly, to the generation of a matrix of uniform fluid droplets from a linear array of fluid jets for use in printing apparatus such as ink jet printing devices and the like.
2. Prior Art
Uniform fluid filaments and synchronous droplet generation is particularly useful in multiple ink jet printing apparatus of the type disclosed, for example, in Lyon U.S. Pat. No. 3,739,393, although the present invention is a different approach of the actual drop stimulation portion of this device. Generally, in such devices there are one or more rows of orifices which receive an electrically conductive recording fluid, such as for instance a water base ink, from a pressurized fluid supply reservoir and eject the fluid in rows of parallel streams or filaments which are stimulated to produce uniform size droplets.
As the droplets are formed they are selectively charged by application of charging voltages to charging electrodes positioned adjacent the filaments at the point where they break up into drops. Droplets which are so charged are deflected by an electrical field into an appropriately positioned catcher. Drops which are not so charged pass through the electrical field without being deflected and are deposited on a web which is transported at relatively high speed across the droplet paths. In addition to achieving maximum printing quality it is important to achieve maximum printing width. In order to achieve the latter, it is essential that there is minimum energy fluctuation throughout the jet array. This energy uniformity is reflected as filament length uniformity within the array. Excessive energy fluctuation (filament length variation) will cause either the generation of satellite droplets or nonlinear behavior of the jet, both of which are unacceptable conditions for printing.
Printing information is transferred to the droplets through charging. In order to print at the highest possible resolution, charging voltages should be applied to the charging electrodes at the same frequency as that at which the drops are being generated. This permits each depositing drop to define a resolution cell distinct from that of all other drops. In addition, printing information cannot be transferred to the drops properly, unless each charging electrode is activated in phase with drop formation at the associated filament. Failure to do this results in partially charged drops, which miss the catcher and deposit at erratic positions on the web.
It is therefore apparent that jet drop printers of the above described type cannot be operated at their maximum capability unless the drops in all stream are generated in synchronism with their associated data transfer charging pulses. This in turn implies either a measurement of drop generation timing for each and every filament or control of drop generation in such a way that the timing or phase of drop generation is predetermined.
The ideal solution from a simplicity point of view is to apply drop stimulating disturbances to all filaments at a common amplitude and in exact synchronism. Then if the jets all have the same diameter, velocity and rheological characteristics, all filaments will have the same length and will generate drops in synchronism. Such synchronized drop generation greatly facilitates the desired data phase locking, because a timing measurement for one jet is a timing measurement for all.
In the above mentioned Lyon et al patent drop generation is accomplished by a traveling wave technique. This method is limited in both maximum printing width and printing quality. As taught by Lyon et al. a series of traveling waves propagate along the length of the orifice plate, stimulating the jets as they go. However, wave propagation is accompanied by energy attenuation. This causes a steady lengthening of the jet filament along the array. Eventually the filament length variation becomes excessive and the maximum usable printing width is reached. The reason the traveling wave method is also limited in printing quality is because in this system the different jets do not generate drops simultaneously, but there is a known phase relation between them.
Thus the system can in theory operate at better resolution, but each data channel must be provided with a phase shifting network for phase shifting the switching control signals by an amount matching the known jet-to-jet drop generation phase shift. This requires a great deal of electronics and is difficult to achieve in practice due to unpredictable variation of plate wavelength (and hence phase errors) caused by nonuniform orifice plate boundries. Even if such synchronization is achieved, the best printing quality is still not available due to the fact that a square droplet matrix can not be formed by traveling waves. Thus such systems have in the past been operated at one-fourth to one-fifth the maximum theoretical resolution; that is, the data frequency and the drop stimulation frequency are so adjusted that three to five drops are generated during one data period. As a result a single resolution cell on the web comprises three to five drops, but it is not necessary to observe any particular phase relation between drop stimulation and drop charging.
An alternative drop stimulation method, which is said to generate uniform filaments and drops simultaneously in a row of jets, is disclosed in Titus et al. U.S. Pat. No. 3,900,162. This patent generally discloses the use of an orifice plate disposed on the bottom of an ink reservoir with the pressure fluctuations in the ink issuing from the orifices being induced by a flexible pressure plate disposed remote from the surface of the orifice plate, but within the ink reservoir.
A plurality of flexible piezoelectric transducers are bonded to the surface of the pressure plate so that when they are simultaneously activated they will produce generally uniform transverse bending along the entire length of the plate so as to produce a uniform pressure distribution in the fluid above the orifices and thus uniformity in the filament and droplet size issuing from the orifices. However, such device requires substantial numbers of the piezoelectric transducers to be bonded along the length of the pressure plate. This results in several practical difficulties due to bonding problems and the cost associated with the relatively high number of transducers required to create uniform stimulation along a long pressure plate.
A further disadvantage associated with such prior art devices is that a slit is built into each end of the plate and the pressure plate is contained within the reservoir itself so that the fluid lies on both sides of the plate. This arrangement does have the advantage of equalizing the pressure on both sides of the pressure plate so that it will bend uniformly in both directions as it vibrates and flexes back and forth. However, it has the unfortunate disadvantage of also producing secondary waves in the fluid passing through the orifices since the fluid on both sides of the pressure plate is in communication and as the pressure plate moves outwardly from the orifices it produces a maximum pressure in the fluid on the side of the pressure plate opposite the orifices which is transmitted through the liquid back to the orifice side. This causes undesirable disturbances in the filaments thus reducing uniformity in filament and droplet size as well as reducing efficiency of energy transmission from the pressure plate to the orifices.
Moreover, the pair of slits can cause two problems. In the first place they can generate reflections of the bending motion and thus cause energy non-uniformity along the plate. Secondly, the fact that they can relieve the pressure in the fluid makes the perturbation by the plate on the fluid ineffective.